[tools] Flush stdout before reading input

The last line of output (which is not terminated by a newline) was not
showing for me when running the merge script. We can either fix it by
specifying `flush=True` at the `print` statement, or flushing before
reading user input. The latter seems more future-proof.

R=machenbach@chromium.org

Change-Id: I61cb929d2f7cdd20b3e32b9beb1653fe2d5c5791
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3676857Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80890}

tools/release/common_includes.py

@@ -146,6 +146,7 @@ class SideEffectHandler(object):  # pragma: no cover
     return Command(cmd, args, prefix, pipe, cwd=cwd)
 
   def ReadLine(self):
+    sys.stdout.flush()
     return sys.stdin.readline().strip()
 
   def ReadURL(self, url, params=None):
@@ -693,8 +694,12 @@ def MakeStep(step_class=Step, number=0, state=None, config=None,
   except AttributeError:
     message = step_class.__name__
 
-    return step_class(message, number=number, config=config,
-                      state=state, options=options,
+  return step_class(
+      message,
+      number=number,
+      config=config,
+      state=state,
+      options=options,
       handler=side_effect_handler)